Fancy, but so devoid of reason (8)
I believe the answer is:
wantonly
'devoid of reason' is the definition.
The answer and definition are not the same part of speech.
'fancy but' is the wordplay.
'fancy' becomes 'want' (both can mean to desire).
'but' becomes 'only' (I've seen this in another clue).
'want'+'only'='WANTONLY'
'so' acts as a link.
Can you help me to learn more?
(Other definitions for wantonly that I've seen before include "in an immoral way" , "In a reckless unrestrained way" , "Arbitrarily" , "In irresponsible way, lacking proper restraint" , "in lascivious way" .)